The invention relates to manufacture and testing of components, namely hammers, for a piano.
Ever since pianos were first developed in the late seventeenth century, manufacture of pianos has been a skilled labor intensive job. As with other jobs performed by skilled craftsmen, "advancement" was slow and the final tone of the finished piano was often strictly dependent upon the ear (hearing) and skill of the craftsman (voicer) involved.
The piano began its history under the name of the pianoforte. This name came from the instrument's unusual ability as a keyed, string instrument to play tones both soft (piano) and loud (forte). The ability to play variations in volume was also one of the main driving forces behind the instrument's development, the other was the increasing desire of the times to play fuller, less delicate music. Many musicians of the time were unsatisfied by the strong, but unemotional, uniform tone produced by the harpsichord in which fairly large strings were plucked by a quill which was controlled by the instrument's keyboard. They were also dissatisfied by the clavichord which produced tone by striking brass hammers against strings and allowed for variations in the volume of play, but was very weak and tinny due to the small thin strings and the brass on steel percussion.
The result of the above dissatisfaction was the invention at about the same time of several different versions of the pianoforte. The most widely acknowledged successful version was a pianoforte developed by Bartolomo Christofori who published details of the instrument in 1711 and completed a prototype around 1720. Among the important innovations contained in Christofori's pianoforte were the hammers which were Wooden blocks covered on the striking surface with soft leather.
Christofori's first compliant piano hammer was simply a polygonal block of wood covered with soft leather on the striking surface as is illustrated in FIG. 1. The hammer was improved over the next several years with the shape being changed to a wedge shaped piece of wood over the thin end of which a piece of leather was bent and attached to give the piano hammer the same basic shape that it has today. Two layers of leather were used in this design with the inner layer being a firm sole leather, and the outer layer being a softer sheepskin. The final manifestation of the leather covered piano hammer used three layers of leather of varying firmness with the softest leather on the outside and the firmest layer on the inside. All of these attempts were aimed toward providing a piano hammer that was soft and compliant at the surface to provide excellent tone for pianissimo playing while the hammer was firmer underneath to provide the strength for forte playing.
After the invention of the iron frame for the strings on a piano, heavier strings could be used at higher tensions to produce a fuller sound from the piano. This rendered the leather covered piano hammer unacceptable. The result was the development and patenting of a felt covered piano hammer by Alpeus Babcock in 1833. These hammers provided a more acceptable tone than the previously discussed leather covered hammers and gained popularity, thus spurring the development of the felt industry and producing patents for the first machine made felt. Felt is a fabric that is formed of hair and/or wool whose fibers are encouraged to interlace through the use of heat, moisture, rolling, beating and pressure. Felt manufacturers experimented with different raw materials and eventually settled on the wool from the merino sheep of Cape Colony in Africa which remained the hammer felt of choice for many years. Eventually, machines were invented to cover the raw wood piano hammers with felt. But, the process of fine tuning the tone of the piano by adjusting the hardness of the hammer felt has remained a skilled craft even to today. This process of tonally regulating the piano hammers is commonly called voicing. It is one of the final steps in the manufacture of a piano, and it is also one of the more time consuming operations.
After the initial step of making sure that the piano hammer felt is properly shaped for the hammer in question, particularly that it is flat across the width of the striking surface, the voicer (a person performing the tonal regulation) typically plays all the notes on the piano several times at varying volume levels to judge the overall tone of the instrument. While doing this the voicer also listens for individual notes which stand out as being too soft (producing a muted or mushy sound) or notes which appear to be too hard (producing an excessively loud or tinny sound).
The voicer adjusts the tone of a hammer that sounds too soft by applying a solution of lacquer and lacquer thinner to the shoulders of the hammer felt of a modern piano hammer, e.g. as shown in FIG. 2 and also as described in Lombino U.S. Pat. No. 5,125,310, issued Jun. 30, 1992 (U.S. Ser. No. 07/657,882, filed Feb. 19, 1991), the disclosure of which is incorporated herein by reference. The shoulder of the hammer is the portion of the hammer adjacent to the striking surface. A method used to harden the striking surface is to iron the surface of the felt with a steam iron and thus encourage the felt fibers to intertwine themselves more tightly.
Hammers that appear to be too hard are adjusted by the voicer through the use of needles (usually three needles oriented axially in the end of a holder) in a process called needling. In order to properly needle the piano hammer felt, the voicer determines whether the hammer is too hard overall, or just at specific volume levels. If the hammer is too hard at very soft (pianissimo) levels, the voicer lightly needles the striking surface of the hammer to a depth of one to two millimeters. However, if the hammer appears too hard at forte or greater levels, then the shoulders of the hammer are deep needled. Where to deep needle the shoulders of the hammer also depends on the volume at which the tone is being adjusted, with the louder volumes causing the needling to be done further away from the striking surface.